There is a pervasive and continuing need for protecting metals from corrosive chemical action, such as in metal pipes, stacks, chimneys, bridges, chemical plant machinery, ship hulls, and containers for aggressive chemicals, to name just a few. In addition to having a high resistance to chemical action, an ideal coating has other properties, specifically: the raw materials required to produce the coating are commercially available, inexpensive, and non-hazardous, the coating has a low viscosity before curing, the hardening process of the coating is quick and does not release pollutants, and the final coating does not contain unnecessary substances.
The most widespread anticorrosive coatings possessing many of the above properties are polyurethanes and epoxide resins (see for example, Coating Systems: A guidance Manual For Field Surveyors, American Bureau of Shipping and Affiliated Companies, 1995). These coatings have good chemical resistance to many substances, have adhesion to metals that is satisfactory for many purposes, and have good mechanical properties. Neither polyurethanes nor epoxide resins, however, satisfy all the criteria for an ideal coating for metal.
As is known from rubber chemistry (Encyclopedia of Polymer Science & Technology, John Wiley & Sons, N.Y., vol 12, p.161, 1970), solid ebonite, commonly known as hard rubber, is a polymer material with sulfur content used for vulcanization. Ebonite, like elastomeric or flexible rubber, is made from a combination of sulfur with polydienes (unsaturated rubbers containing double bonds). Solid ebonite is a hard, non-flexible, plastic-like material possessed of unique chemical resistance to aggressive substances such as acids, alkalis, salt solutions, oil, and gasoline. In addition, solid ebonite has good mechanical properties. In spite of these advantages, however, solid rubbers have a high viscosity and can not be easily applied to metal surfaces, they release toxic fumes during vulcanization, and they require a long time to harden.
A liquid ebonite formulation for coating metals was developed by Ju. N. Pushkarev, V. I. Anosoff, and A. L. Labutin, and described in their article "A vulcanization and the properties of ebonite coatings made on the base of liquid CiS-1.4-polybutadiene", Caoutchouc and Rezina, N3, 1979 (Russian). The formulation is prepared by mixing the components as shown in Table 1.
TABLE 1 ______________________________________ Compound Mass Parts ______________________________________ Liquid polydiene rubber with hydroxyls 100 or without functional groups Sulfur 40 diphenylguanidine (accelerator) 3 Zinc Oxide (activator) 5 Cab-O-Sil (thixotropy agent) 5 white spirit (thinner; used sometimes) 5-10 ______________________________________
The curing processes of liquid ebonite has presented problems in the past. During the process volatile and toxic products are released, including hydrogen sulfide. Moreover, the release of gases during curing produces microcavities which reduce the strength and protective properties of the coating.